[C++20] [Modules] Don't load declaration eagerly for named modules

Close #61064.

The root cause of the issue is that we will deserilize some declarations
eagerly when reading the BMI. However, many declarations in the BMI are
not necessary for the importer. So it wastes a lot of time.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -2468,7 +2468,13 @@ void ASTWriter::WriteDeclAbbrevs() {
 /// relatively painless since they would presumably only do it for top-level
 /// decls.
 static bool isRequiredDecl(const Decl *D, ASTContext &Context,
-                           bool WritingModule) {
+                           Module *WritingModule) {
+  // Named modules have different semantics than header modules. Every named
+  // module units owns a translation unit. So the importer of named modules
+  // doesn't need to deserilize everything ahead of time.
+  if (WritingModule && WritingModule->isModulePurview())
+    return false;
+
   // An ObjCMethodDecl is never considered as "required" because its
   // implementation container always is.
 

clang/test/Modules/no-eager-load.cppm

@@ -0,0 +1,110 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/a.cppm -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/b.cppm -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/c.cpp \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/d.cpp \
+// RUN:     -fprebuilt-module-path=%t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/e.cppm -o %t/e.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/f.cppm -o %t/f.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/g.cpp \
+// RUN:     -fprebuilt-module-path=%t
+//
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/h.cppm \
+// RUN:     -fprebuilt-module-path=%t -o %t/h.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface %t/i.cppm \
+// RUN:     -fprebuilt-module-path=%t -o %t/i.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/j.cpp \
+// RUN:     -fprebuilt-module-path=%t
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %t/k.cpp \
+// RUN:     -fprebuilt-module-path=%t
+
+//--- a.cppm
+export module a;
+export void foo() {
+
+}
+
+//--- b.cppm
+export module b;
+void bar();
+export void foo() {
+    bar();
+}
+
+//--- c.cpp
+// expected-no-diagnostics
+// Since we will load all the declaration lazily, we won't be able to find
+// the ODR violation here.
+import a;
+import b;
+
+//--- d.cpp
+import a;
+import b;
+// Test that we can still check the odr violation if we call the function
+// actually.
+void use() {
+    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
+           // expected-note@* {{but in 'a' found a different body}}
+}
+
+//--- foo.h
+void foo() {
+
+}
+
+//--- bar.h
+void bar();
+void foo() {
+    bar();
+}
+
+//--- e.cppm
+module;
+#include "foo.h"
+export module e;
+export using ::foo;
+
+//--- f.cppm
+module;
+#include "bar.h"
+export module f;
+export using ::foo;
+
+//--- g.cpp
+import e;
+import f;
+void use() {
+    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
+           // expected-note@* {{but in 'e.<global>' found a different body}}
+}
+
+//--- h.cppm
+export module h;
+export import a;
+export import b;
+
+//--- i.cppm
+export module i;
+export import e;
+export import f;
+
+//--- j.cpp
+import h;
+void use() {
+    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
+           // expected-note@* {{but in 'a' found a different body}}
+}
+
+//--- k.cpp
+import i;
+void use() {
+    foo(); // expected-error@* {{'foo' has different definitions in different modules;}}
+           // expected-note@* {{but in 'e.<global>' found a different body}}
+}
+